Method of making glass

ABSTRACT

A method of making glass in which a first glass composition is fed to a furnace and the densities of such glass is determined. The composition is then changed to produce a second glass which has substantially different properties from the first glass, the change to the composition being such that the desired properties are produced in the second glass while simultaneously maintaining the densities of the second glass substantially equal to that of the second glass. The first glass may be a clear glass while the second glass may be a tinted glass. In such a case, the tinted glass may contain iron and the densities are maintained substantially equal by utilizing the iron to replace calcium oxide in the clear glass composition.

FIELD OF THE INVENTION

The present invention relates to a method of making glass. Moreparticularly, the present invention relates to a method of making glasspreferably, but not essentially, by the float process which permits thechangeover time from making a clear glass to a tinted glass and viceversa to be considerably shortened.

BACKGROUND OF THE INVENTION AND PRIOR ART DISCUSSION

It is well known to make both clear and tinted glasses by the floatprocess. At the present time, tinted glasses are much used in automotivevehicles. Such tinted glasses are not provided simply for aestheticreasons. Clear glass does, of course, provide excellent luminousradiation transmission but has the consequential drawback of permittingother, unwanted radiation to enter the interior of the vehicle. Thussolar heat radiation, in both the visible and infra-red regions of theelectromagnetic spectrum, can cause the temperature in the interior ofthe vehicle to become sufficiently high as to cause discomfort to theoccupants and/or damage to goods being transported. Moreover, clearglass allows a high proportion of ultraviolet radiation to passtherethrough and it is well known that such radiation causes theembrittling of plastics materials and the fading of the colours offabrics in the interior of the vehicle.

To alleviate these problems, it is well known to add colorants to thebasic, clear glass composition. In particular, it is extremely wellknown to employ iron oxide as the colorant Iron oxide (Fe₂ O₃) isparticularly beneficial because it contains iron in both the ferric andferrous states. Ferric iron reduces the amount of ultraviolet radiationthat is transmitted through the glass whilst ferrous iron absorbsradiation in the near infra-red region at about 1050 nm.

However, the use of iron oxide does have disadvantageous side effects.In particular, it increases the densities of the glass. The most obviousdrawback of this is that the weight of the vehicle in which the glass isincorporated is increased. From a manufacturing point of view,particularly on a float line, a far more serious problem manifestsitself. For commercial reasons float lines are not usually dedicated tomaking one particular type of glass. For example, a float line may beused for making clear glass, then for making a tinted glass and thenrestored to making clear glass. It will be readily appreciated that thechangeover cannot be effected instantaneously.

When a changeover from clear glass to a tinted glass is made, the tintedglass will not be of the desired quality until all of the colorant isuniformly incorporated in, that is to say, mixed into, the glass. Ittakes longer for this to be achieved if certain colorant materials, suchas iron oxide, are used. Similarly, when the reverse changeover is made,the clear glass must be completely clear. In other words, there must notbe any streaks of colour in the glass.

When the changeover from clear to tint is effected, the changeover isretarded by the fact that the colorant has a higher densities than themolten clear glass and tends to sink to the bottom of the melter. Themixing of the colorant with the clear glass constituents is, therefore,somewhat inefficient and takes a considerable length of time to optimiseitself. Until optimal mixing is attained, the tinted glass will not beof an acceptable commercial quality. When the run of making tinted glassis completed, the colorant again tends to settle on the bottom of thetank and is only removed slowly therefrom. These changeover times,particularly the changeover from tinted glass to clear glass can be verylong, of the order of weeks. From a commercial point of view, thisrepresents a significant disadvantage.

A further problem is that of devitrification. When making a tintedglass, particularly one containing iron oxide, there is a greatertendency for a build-up of devitrified glass on the bottom of thefurnace at the working end to occur.

The problems of incomplete mixing and devitrification are, inglass-making terms, problems on a macro-scale. There is also aminor-scale problem which occurs when the composition of the glass ischanged, such as when there is a change-over in a furnace from makingone kind of glass to a second kind of glass. This is the problem ofchanges occuring in the refractive index within the glass being formedand is commonly known as "ream". This manifests itself in the form ofoptical distortion when one looks through the glass.

Still further, and alluded to above, vehicle manufacturers require theglass which they use in their vehicles to have ever lower weight butstill to have the particular radiation transmission or absorptionproperties. Accordingly, glass manufacturers are attempting to makethinner glass having the desired properties. According to theLambert-Beer law, the thickness of a glass (or the concentration of acolorant species) is linked with the visible transmission of the glass.In essence, this states that as the thickness of the glass is halved,the internal optical density of the glass will also halve. Accordingly,in order to make a glass having a thickness of 2 mm having the sameradiation transmission or absorption properties as a glass having athickness of 4 mm, it is usually necessary to double the concentrationof the colorant. Of course, if the concentration of the colorant isdoubled, the problems outlined above caused by the higher densities ofthe colorant compared with that of the clear glass composition areexacerbated.

OBJECTS OF THE INVENTION

The present invention therefore seeks to provide a method of makingglass in which the above-described problems are at least minimized. Inparticular, the present invention seeks to provide a method of makingglass so that the changeover time when a float line is changed frommaking a first type of glass to a second type of glass is substantiallyreduced.

SUMMARY OF THE INVENTION

According to the present invention, there is therefore provided a methodof making glass comprising the steps of producing a first glass in afurnace, measuring the densities of the glass thus produced and alteringthe composition fed to the furnace such that a further glass havingsignificantly different properties from the first glass is produced,wherein the alteration to the composition is so selected as to producethe desired properties in the further glass whilst simultaneouslymaintaining the densities of the further glass substantially equal tothat of the first glass.

We have surprisingly found that if the compositions of the differentglasses made in a particular furnace are selected so that the densitiesof all of the glasses made in that furnace, measured at 25° C., aresubstantially the same, the changeover from making one commerciallyacceptable glass to another commercially acceptable glass can beeffected much more rapidly. The term "substantially the same densities"is defined as meaning that the densities are within 0.01, preferablywithin 0.005, of one another. Moreover, we have found that if thedensities of the different glasses are matched, the refractive indicestend also to be more closely matched. Accordingly, we have found that,by carrying out the method of the present invention, the problem of reammay also be alleviated.

Preferably, the first glass is a clear glass and the further glass is atinted glass containing iron.

Alternatively, both said first and second glasses contain Fe₂ O₃characterised in that the densities of the two glasses are madesubstantially constant by reducing the CaO content in the glasscontaining more Fe₂ O₃ relative to the glass containing less Fe₂ O₃.

Glass used in vehicles and buildings is usually of the soda-lime-silicatype and the major constituents thereof are SiO₂, Na₂ O, CaO, MgO, K₂ Oand Al₂ O₃. Of these, the lightest component is SiO₂ which, when fused,has a densities of approximately 2.2. Most glass manufacturers have abasic clear glass composition which they maintain substantiallyconsistent. If they then wish to make a tinted glass containing iron,this is effected in one of two ways. Firstly the composition may bemodified by selecting the appropriate amount of iron oxide to be added(in weight %) and reducing the amount of SiO₂ present in the compositionby the same amount. Alternatively, the iron oxide may simply be added tothe other constituents of the clear glass composition. It will bereadily apparent that, if the amounts are calculated in weight %, thislatter procedure has the effect of slightly reducing the weightpercentage of all of the components originally present

We have found that each of the constituents of a glass making mixturehave an effect on the densities of the glass which is produced. Whilstthis may appear, at first sight, to be self-evident, we havesurprisingly found that the effects produced are not predictable. Forexample, the densities of iron oxide, in the form of Fe₂ O₃, and CaO areapproximately 5.2 and 3.2 respectively. However, for each weight percentof Fe₂ O₃ used to replace one weight percent of SiO₂, the densities ofthe glass rises by approximately 0.0150. For each weight percent of CaOused to replace one weight percent of SiO₂, the densities rises by0.0149. This is totally unexpected on the basis of the densities of theFe₂ O₃ and the CaO. Similarly, from their densities, one would expect K₂O to have a greater effect on the density of the glass than Na₂ O.However, again referring to the replacement of one weight percent ofSiO₂ by one weight percent of each of these alkali metal oxides, we havefound that the sodium oxide increases the densities of the glass by0.0078 whilst the potassium oxide only increases it by 0.0056.

From the above, it will be readily apparent that if the tinted glass isto contain Fe₂ O₃ in relatively large amounts--say from 0.7 to 5 weight%--the densities of the glass would be substantially increased if theusual procedure of reducing the amount of SiO₂ in an amountcorresponding to that of the iron oxide added were adopted. However, itwill be equally apparent from the above discussion that if the amount ofCaO is reduced by an amount corresponding to the amount of Fe₂ O₃ added,the effect on the densities will be very small (an increase ofapproximately 0.0001 per weight percent of Fe₂ O₃ added).

However, particularly if there is a large amount of iron in the tintedglass, a straight exchange of Fe₂ O₃ for CaO can give rise to problems.In particular, although lowering the amount of CaO lessens the chance ofdevitrification due to wollastonite (calcium silicate) occurring, toolow an amount of calcium present tends to encourage the formation ofphases such as diopside or silica-rich primary phases such as tridymite.Both of these are harder to remove than wollastonite, particularly thesilica-rich phases.

Accordingly, it may be preferred if the MgO content is simultaneouslyreduced along with the CaO. The removal of calcium oxide and/ormagnesium oxide can lead to a change in the working temperature (alsoknown as the log4 viscosity because this is the temperature at which theglass has a viscosity of 10⁴ poise). Since a change in the workingtemperature is not usually regarded as desirable by glass manufacturersbecause it may necessitate physical changes to other parameters it maytherefore be desirable if this is counterbalanced by a slight increasein the amount of Na₂ O and K₂ O in the composition.

In a particularly preferred embodiment of the present invention, thefirst glass contains A% CaO, B% MgO, C% Na₂ O plus K₂ O and D% SiO₂ plusAl₂ O₃, and the second glass contains X% total iron (expressed as Fe₂O₃), (A-0.5X)% to (A-1.5X)% CaO, B% to (B-X)% MgO with the proviso thatthe amount of MgO tends to (B-X) when the change in CaO is least, C to(C+0.5X) Na₂ O plus K₂ O and D% SiO₂ plus Al₂ O₃ with sufficient changesto cause the total contents to be 100 weight %.

As previously mentioned, the tinted glasses are intended, in most casesto be so-called "high performance glasses", that is to say, they havelow infra-red and ultraviolet radiation transmittance. In a particularlypreferred embodiment of the present invention, the glasses are primarilyintended for use as either privacy glazing or as the glazing for asunroof in a vehicle. To produce such glazing, the iron content in thecomposition needs to be relatively high and the method of the presentinvention is particularly useful in preparing glass wherein the ironoxide content is in excess of 0.7 weight percent and may be as high as 5weight percent.

Other colorants may be used in association with the iron oxide. Examplesof such colorants are selenium, nickel oxide and cobalt oxide. Eventhough these have relatively high molecular weights, the amounts whichare used are relatively small, of the order of 500 ppm or less and suchsmall amounts have such a small effect on the densities of the finishedglass that, for the practical purposes of the present invention, theycan be ignored.

If the amount of calcium oxide is reduced as the amount of iron oxide isincreased, the working temperature of the glass remains substantiallythe same. However, the liquidus temperature, that is to say, thetemperature at which the glass will begin to crystallise underequilibrium conditions, will drop. This is because magnesium calciumsilicate (diopside) is then formed as the primary or highest temperaturephase as the amount of calcium in the glass is reduced. This aspect ofthe present invention therefore helps to prevent a build-up ofdevitrified glass. Devitrification is a common problem in glassescontaining high amounts of iron oxide. The working range, which is thedifference between the working temperature and the liquidus temperature,is thereby increased. This latter is of importance because it means thatit is easier to form the glass without crystallisation occurring and theamount of devitrification is reduced or even eliminated completely.

In another aspect of the present invention, there is provided a highsoda-lime-silica glass for the production of vehicle windows having alow Direct Solar Heat Transmission by virtue of the presence ofcolorants including the iron, characterised in that the compositioncontains a reduced proportion of alkaline earth metal oxides. In apreferred embodiment, the glass contains more than 2.0 weight % ironcalculated as Fe₂ O₃ and less than 12 weight % alkaline earth metaloxides. Advantageously, the alkaline earth metal oxide content is below11% and, desirably, the iron content is less than 2.5 weight %,calculated as Fe₂ O₃. Glasses of this type preferably have a workingrange of at least 9C.

Although high iron content glasses have been made hitherto, such glasseshave either not had the desired optical properties or have causedproduction difficulties of the types detailed hereinbefore. We havesurprisingly found that, by reducing the amount of alkaline earth metaloxides present in high iron glasses to below, for example 12%, theproduction difficulties are overcome whilst the optical properties areattained.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will be further described, by way of example only, withreference to the following non-limitative Examples appearing in thefollowing Tables. In the tables:

The ferrous state is the amount of FeO (expressed as Fe₂ O₃), divided bythe total iron oxide (expressed as Fe₂ O₃).

Illuminant A transmission represents the power distribution of tungstenfilament according to the 1931 Observer set.

DSHT (Direct Solar Heat Transmission) refers to solar heat transmissionintegrated over the wavelength range 350 to 2100 nm according to therelative solar spectral distribution Parry Moon for air mass 2 direct,as detailed in ISO 9050.

UVT (ISO) for the measurement of ultraviolet radiation over thewavelength range 280-380 nm (air mass 2 global) is from standard ISO9050.

UVT (PM) is the measurement of ultraviolet radiation over the wavelengthrange 300 to 400 nm derived from the factors published by Parry Moon inthe Journal of the Franklin Institute 230 pp 583-617, 1940.

a* and b* are colour co-ordinates derived using the CieLAB co-ordinatesystem.

The log 4 viscosity temperature is the temperature in degrees Celsius atwhich the glass has a viscosity of 10,000 poise.

The liquidus temperature is determined by placing glass grains in agradient furnace for 16 hours then measuring the hottest temperature atwhich crystal growth occurs.

Density is the mass per unit volume in grams per cubic centimeter at 25°C. measured using the Archimedes method.

W=Wollastonite

Di=Diopside

De=Devitrite

Si=Silica

Nd=Refractive Index

                                      TABLE 1                                     __________________________________________________________________________    Example 1   2a  2b  3   4   5   6   7                                         __________________________________________________________________________    SiO.sub.2                                                                             72.7                                                                              71.77                                                                             71.4                                                                              72.7                                                                              72.7                                                                              72.2                                                                              72.7                                                                              72.7                                      Al.sub.2 O.sub.3                                                                      1.0 0.987                                                                             1.0 1.0 1.0 1.0 1.0 1.0                                       Na.sub.2 O                                                                            13.0                                                                              12.833                                                                            13.0                                                                              13.0                                                                              13.0                                                                              13.5                                                                              13.0                                                                              13.0                                      K.sub.2 O                                                                             0.6 0.592                                                                             0.6 0.6 0.6 0.6 0.6 0.6                                       MgO     4.0 3.949                                                                             4.0 4.0 3.8 3.8 3.4 2.8                                       CaO     8.4 8.292                                                                             8.4 7.2 7.4 7.4 7.8 8.4                                       SO.sub.3                                                                              0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2                                       Fe.sub.2 O.sub.3                                                                      0.1 1.3 1.3 1.3 1.3 1.3 1.3 1.3                                       Ferrous state                                                                         20% 22% 22%                                                           III. A Trans                                                                  DSHT                                                                          UVT (ISO)                                                                     UVT (PM)                                                                      a*                                                                            b*                                                                            dom λ nm                                                               Colour Purity %                                                               log4 visc.                                                                            1037                                                                              (1033)                                                                            (1028)                                                                            1037                                                                              1038                                                                              1030                                                                              1034                                                                              1032                                      temp. °C.                                                              Liquidus                                                                              1000 W                                                                            1000 W  982 Di                                                                            982 W                                                                             980 W                                                                             990 W                                                                             998 W                                     Temp. (T.sub.l)                                                                           998 Di  964 De                                                                            976 Di  963 De                                        T.sub.w - T.sub.l                                                                     +36° C.                                                                    +33° C.                                                                    +28° C.                                                                    +55° C.                                                                    +56° C.                                                                    +50° C.                                                                    +44° C.                                                                    +34° C.                            Densities                                                                             2.4910                                                                            2.5086                                                                            2.5103                                                                            2.4925                                                                            2.4941                                                                            2.4987                                                                            2.4963                                                                            2.4996                                    Refractive                                                                            1.5180                                                                            1.5224                                                                            1.5230                                                                            1.5188                                                                            1.5188                                                                            1.5194                                                                            1.5197                                                                            1.5207                                    Index (Nd)                                                                    __________________________________________________________________________    Example 8   9   10  11  12  13  14  15                                        __________________________________________________________________________    SiO.sub.2                                                                             71.14                                                                             72.7                                                                              72.7                                                                              72.2                                                                              72.4                                                                              72.7                                                                              72.7                                                                              72.7                                      Al.sub.2 O.sub.3                                                                      0.98                                                                              1.0 1.0 1.5 1.5 1.0 1.0 1.0                                       Na.sub.2 O                                                                            12.72                                                                             13.0                                                                              13.0                                                                              14.1                                                                              14.2                                                                              13.0                                                                              13.0                                                                              13.0                                      K.sub.2 O                                                                             0.587                                                                             0.6 0.6 0.6 1.0 0.6 0.6 0.6                                       MgO     3.91                                                                              3.4 3.4 4.0 1.0 3.6 3.0 0                                         CaO     8.22                                                                              6.8 6.8 5.1 7.4 7.1 5.4 8.4                                       SO.sub.3                                                                              0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2                                       Fe.sub.2 O.sub.3                                                                      2.2 2.2 2.2 2.2 2.2 1.7 4.0 4.0                                       Ferrous state                                                                         22% 22% 30% 22% 33% 33% 22.4%                                                                             21%                                       Co.sub.3 O.sub.4                                                                      295 275 235 285 235 90  500 500                                       Se      15  16  17  24  18  10  20  13                                        NiO     250 250 75  150 0   50  400 400                                       III. A Trans                                                                          28.1                                                                              31.5                                                                              32.0                                                                              28.5                                                                              34.1                                                                              51.5                                                                              11.0                                                                              16.0                                      DSHT    20.9                                                                              24.1                                                                              21.4                                                                              21.1                                                                              22  32.5                                                                              9.0 14.4                                      UVT (ISO)                                                                             4.9 6.6 6.1 6.1 8.7 14.4                                                                              0.5 0.8                                       UVT (PM)                                                                              15.4                                                                              19  17.6                                                                              17.4                                                                              22.6                                                                              32  3.4 5.6                                       a*      -7.0                                                                              -6.4                                                                              -6.3                                                                              -5.8                                                                              -7.1                                                                              -7.6                                                                              -7.7                                                                              -8.9                                      b*      -0.6                                                                              -1.5                                                                              -0.4                                                                              -1.7                                                                              -6.4                                                                              -0.5                                                                              +2.2                                                                              -2.8                                      dom λ nm                                                                       493.7                                                                             492 494 491 486 494 507 491                                       Colour Purity %                                                                       5.7 6   4.8 6.1 11.4                                                                              4.2 4.5 11.1                                      Log 4 visc.                                                                           (1027)                                                                            1042                                                                              1042                                                                              1038                                                                              1012                                                                              (1040)                                                                            1044                                          temp ° C.                                                              Liquidus                                                                              1016 Di                                                                           970 970 990 Di                                                                            958 De                                                                            972 980 Si                                        Temp. (T.sub.l)                                                                           W/Di                                                                              W/Di        W/Di                                              T.sub.w - T.sub.l                                                                     +11° C.                                                                    +72° C.                                                                    +72° C.                                                                    +48° C.                                                                    +54° C.                                                                    +68° C.                                                                    +64° C.                                Density 2.5189                                                                            2.4951                                                                            2.4965                                                                            2.4884                                                                            2.4995                                                                            2.4944                                                                            2.4983                                                                            2.5153                                    Refractive Index                                                                      1.5253                                                                            1.5197                                                                            1.5197  1.5202                                                                            1.5194                                                                            1.5216                                                                            1.5259                                    __________________________________________________________________________    Example 16   17  18   19  20   21  22                                         __________________________________________________________________________    SiO.sub.2                                                                             72.0 72.7                                                                              72.7 72.7                                                                              72.7 72.7                                                                              72.7                                       Al.sub.2 O.sub.3                                                                      1.9  1.0 1.0  1.0 1.0  1.0 1.0                                        Na.sub.2 O                                                                            15.0 13.0                                                                              13.0 13.0                                                                              13.0 13.0                                                                              13.0                                       K.sub.2 O                                                                             0.6  0.6 0.6  0.6 0.6  0.6 0.6                                        MgO     0    4.0 3.45 3.8 3.7  3.6 3.7                                        CaO     6.2  4.4 6.8  7.5 7.4  7.1 7.4                                        SO.sub.3                                                                              0.2  0.2 0.2  0.2 0.2  0.2 0.2                                        Fe.sub.2 O.sub.3                                                                      4.0  4.0 2.15 1.1 1.3  1.7 1.3                                        Ferrous state                                                                         20.5%                                                                              21% 26%  27% 23%  34% 27%                                        Co.sub.3 O.sub.4                                                                      500  500 200  113 130  170 130                                        Se      43   12  26   19  15   21  10                                         NiO     400  400 75   50  50   75  50                                         III. A Trans                                                                          9.3  16.7                                                                              32.5 52  52.2 38.9                                                                              53.4                                       DSHT    8.8  15.1                                                                              23.4 43  43   26  41.5                                       UVT (ISO)                                                                             0.5  0.6 5.4  18  16   11  18.4                                       UVT (PM)                                                                              3.2  4.6 16   36.2                                                                              34.6 24.1                                                                              37.7                                       a*      -2.8 -10.0                                                                             -3.9 -1.6                                                                              -3.2 -5.5                                           b*      5.9  -0.2                                                                              +4.9 +3.2                                                                              +0.8 +0.9                                           dom λ nm                                                                       568  495.8                                                                             561  586 498  498 490                                        Colour Purity %                                                                       10.7 8.4 5.1  2.7 1.5  3   4.2                                        log4 visc.                                                                            1005                                                                  temp. °C.                                                              Liquidus                                                                              955 De                                                                Temp. (T.sub.l)                                                                       950 Si                                                                T.sub.w - T.sub.l                                                                     +50° C.                                                        Densities                                                                             2.5074                                                                             2.4899                                                                            2.4933                                                                             2.4938                                                                            2.4941                                                                             2.4947                                                                            2.4944                                     Refractive Index                                                                      1.5214                                                                             1.5202                                                           __________________________________________________________________________    Example 23   24   25   26   27   28                                           __________________________________________________________________________    SiO.sub.2                                                                             72.7 72.7 72.7 72.7 72.7 72.7                                         Al.sub.2 O.sub.3                                                                      1.0  1.0  1.0  1.0  1.0  1.0                                          Na.sub.2 O                                                                            13.0 13.0 13.0 13.0 13.0 13.0                                         K.sub.2 O                                                                             0.6  0.6  0.6  0.6  0.6  0.6                                          MgO     3.6  3.7  3.8  3.6  3.6  3.4                                          CaO     7.1  7.4  7.5  7.1  7.1  6.8                                          SO.sub.3                                                                              0.2  0.2  0.2  0.2  0.2  0.2                                          Fe.sub.2 O.sub.3                                                                      1.7  1.3  1.1  1.7  1.7  2.2                                          Ferrous state                                                                         27%  27%  28%  36.5%                                                                              37.5%                                                                              28.4%                                        Co.sub.3 O.sub.4                                                                      10   45   13   170  170  255                                          Se      56   26   31   10   5    12                                           NiO     75   50   50   75   75   150                                          III. A Trans                                                                          35   53   56   43.3 44.7 32.7                                         DSHT    27   39   43   28   28.6 23                                           UVT (ISO)                                                                             5.3  13.3 15   13.9 15.4 7                                            UVT (PM)                                                                              12   29.5 31.5 31   33.4 19.6                                         a*      +8.0 -1.0 +1.4 -7.9 -8.7 -7.7                                         b*      +26.9                                                                              +10.0                                                                              +13.4                                                                              -5.4 -7.3 -4.0                                         dom λ nm                                                                       582  575  578  488  487  489                                          Colour Purity %                                                                       38.6 11   16   10.0 12.1 9.3                                          log4 visc.                                                                    temp. °C.                                                              Liquidus               976 W                                                  Temp. (T.sub.l)        970 Di                                                 T.sub.w - T.sub.l                                                             Densities                                                                             2.4940                                                                             2.4949                                                                             2.4944                                                                             2.4961                                                                             2.4955                                                                             2.4967                                       Refractive index                                                              __________________________________________________________________________    Example 29      30      31      32                                            __________________________________________________________________________    SiO.sub.2                                                                             72.6    72.6    70.6    70.6                                          Al.sub.2 O.sub.3                                                                      0.1     0.1     0.91    0.91                                          Na.sub.2 O                                                                            13.9    13.9    13.7    13.7                                          K.sub.2 O                                                                             0.03    0.03    0.57    0.57                                          MgO     4.1     3.5     4.0     3.4                                           CaO     8.9     7.3     9.76    8.16                                          SO.sub.3                                                                              0.23    0.23    0.27    0.27                                          Fe.sub.2 O.sub.3                                                                      0.11    2.2     0.1     2.2                                           Ferrous state                                                                         20%     24%     20%     24%                                           Co.sub.3 O.sub.4                                                                              275             275                                           Se              23              22                                            NiO             250             250                                           III. A Trans    29.4            29                                            DSHT            23.5            23.1                                          UVT (ISO)       5.4             4.5                                           UVT (PM)        16.4            14.6                                          a*              -5.1            -5.4                                          b*              +1.4            +2.7                                          dom λ nm 503             522.2                                         Colour Purity % 2.4             2.1                                           log4 visc.                                                                            1014    1020    1000    1010                                          temp. °C.                                                              Liquidus                                                                              1008 W  996 Si  1028 W  990 W                                         Temp. (T.sub.l) 988 W                                                         T.sub.w - T.sub.l                                                                     +6      +24     -28     +20                                           Densities                                                                             2.5017  2.5017  2.5176  2.5237                                        Refractive index                                                                      1.5201  1.5221  1.5249  1.5268                                        __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Example 1   8   9   10   11  12  13   14                                      __________________________________________________________________________    SiO.sub.2                                                                             72.7                                                                              71.14                                                                             72.7                                                                              72.7 72.7                                                                              72.4                                                                              72.7 72.7                                    Al.sub.2 O.sub.3                                                                      1.0 0.98                                                                              1.0 1.0  1.5 1.5 1.0  1.0                                     Na.sub.2 O                                                                            13.0                                                                              12.72                                                                             13.0                                                                              13.0 14.1                                                                              14.2                                                                              13.0 13.0                                    K.sub.2 O                                                                             0.6 0.587                                                                             0.6 0.6  0.6 1.0 0.6  0.6                                     MgO     4.0 3.91                                                                              3.4 3.4  4.0 1.0 3.6  3.0                                     CaO     8.4 8.22                                                                              6.8 6.8  5.1 7.4 7.1  5.4                                     SO.sub.3                                                                              0.2 0.2 0.2 0.2  0.2 0.2 0.2  0.2                                     Fe.sub.2 O.sub.3                                                                      0.1 2.2 2.2 2.2  2.2 2.2 1.7  4.0                                     Ferrous state                                                                         20% 22% 22% 30%  22% 33% 33%  22.4%                                   Co.sub.3 O.sub.4                                                                          295 275 235  285 235 90   500                                     Se          15  16  17   24  18  10   20                                      NiO         250 250 75   150 0   50   400                                     III. A Trans                                                                              8.7 10.7                                                                              11.2 9   12.9                                                                              29.2 1.4                                     DSHT        5.6 7.1 6.3  5.8 7.2 14.9 1.1                                     UVT (ISO)   0.7 1.1 1.0  0.9 1.9 4.6  0                                       UVT (PM)    3.9 5.5 4.9  4.8 7.8 14.7 0.3                                     a*          -9.1                                                                              -8.7                                                                              -8.6 -7.5                                                                              -9.4                                                                              -12.1                                                                              -7.3                                    b*          -0.6                                                                              -1.9                                                                              -0.4 -2.1                                                                              -9.1                                                                              -0.6 +2.4                                    dom λ nm                                                                           494.9                                                                             492.3                                                                             495.2                                                                              491.4                                                                             486 495.1                                                                              510.7                                   Colour Purity %                                                                           10.1                                                                              11.0                                                                              8.6  10.9                                                                              20.8                                                                              8.9  8.2                                     Log 4 visc.                                                                           1037    1042     1.038                                                                             1012     1044                                    temp °C.                                                               Liquidus                                                                              1000 W                                                                            1016 Di 970 W/Di                                                                           990 Di                                                                            958 De                                                                            972 W/Di                                                                           980 Si                                  Temp. (T.sub.l)                                                               T.sub.w - T.sub.l                                                                     +36° C.   +48° C.                                                                    +54° C.                                   Densities                                                                             2.4910                                                                            2.5189                                                                            2.4951                                                                            2.4965                                                                             2.4884                                                                            2.4995                                                                            2.4944                                                                             2.4983                                  Refractive Index                                                                      1.5180                                                                __________________________________________________________________________    Example 15  16   17   18   19   20   21                                       __________________________________________________________________________    SiO.sub.2                                                                             72.7                                                                              72.0 72.7 72.7 72.7 72.7 72.7                                     Al.sub.2 O.sub.3                                                                      1.0 1.9  1.0  1.0  1.0  1.0  1.0                                      Na.sub.2 O                                                                            13.0                                                                              15.0 13.0 13.0 13.0 13.0 13.0                                     K.sub.2 O                                                                             0.6 0.6  0.6  0.6  0.6  0.6  0.6                                      MgO     0   0    4.0  3.45 3.8  3.7  3.6                                      CaO     8.4 6.2  4.4  6.8  7.5  7.4  7.1                                      SO.sub.3                                                                              0.2 0.2  0.2  0.2  0.2  0.2  0.2                                      Fe.sub.2 O.sub.3                                                                      4.0 4.0  4.0  2.15 1.1  1.3  1.7                                      Ferrous state                                                                         21% 20.3%                                                                              21%  26%  27%  23%  34%                                      Co.sub.3 O.sub.4                                                                      500 500  500  200  113  130  170                                      Se      13  43   12   26   19   15   21                                       NiO     400 400  400  75   50   50   75                                       III. A Trans                                                                          2.9 1.0  2.7  11.6 29.9 29.9 16.6                                     DSHT    2.7 1.1  2.5  7.0  21.6 21.4 9.2                                      UVT (ISO)                                                                             0   0    0    0.8  6.7  5.7  2.6                                      UVT (PM)                                                                              0.6 0.2  0.4  4.1  18   17   8.7                                      a*      -9.5                                                                              -2.5 -10.8                                                                              -5.5 -2.6 -5.3 -8.1                                     b*      -3.2                                                                              5.1  0    +6.9 +5.3 +1.2 +1.4                                     dom λ nm                                                                       491 568  496.8                                                                              561.4                                                                              568  501.2                                                                              500.9                                    Colour Purity %                                                                       20.2                                                                              22.5 15.4 10.9 6.2  2.7  5.1                                      log4 visc.  1005                                                              temp. °C.                                                              Liquidus    955 De                                                            Temp. (T.sub.l)                                                                           950 Si                                                            T.sub.w - T.sub.l                                                                         +50° C.                                                    Densities                                                                             2.5153                                                                            2.5074                                                                             2.4899                                                                             2.4933                                                                             2.4938                                                                             2.4941                                                                             2.4947                                   Refractive Index                                                              __________________________________________________________________________    Example 22  23   24   25   26   27   28                                       __________________________________________________________________________    SiO.sub.2                                                                             72.7                                                                              72.7 72.7 72.7 72.7 72.7 72.7                                     Al.sub.2 O.sub.3                                                                      1.0 1.0  1.0  1.0  1.0  1.0  1.0                                      Na.sub.2 O                                                                            13.0                                                                              13.0 13.0 13.0 13.0 13.0 13.0                                     K.sub.2 O                                                                             0.6 0.6  0.6  0.6  0.6  0.6  0.6                                      MgO     3.7 3.6  3.7  3.8  3.6  3.6  3.4                                      CaO     7.4 7.1  7.4  7.5  7.1  7.1  6.8                                      SO.sub.3                                                                              0.2 0.2  0.2  0.2  0.2  0.2  0.2                                      Fe.sub.2 O.sub.3                                                                      1.3 1.7  1.3  1.1  1.7  1.7  2.2                                      Ferrous state                                                                         27% 27%  27%  28%  36.5%                                                                              37.5%                                                                              28.4%                                    Co.sub.3 O.sub.4                                                                      130 10   45   13   170  170  255                                      Se      10  56   26   31   10   5    12                                       NiO     50  75   50   50   75   75   150                                      III. A Trans                                                                          31.2                                                                              13.7 31.3 34.2 20.8 22.3 11.9                                     DSHT    20.6                                                                              8.9  18.8 21.6 11.4 12.2 7.1                                      UVT (ISO)                                                                             7.1 0.5  4.3  4.9  4.3  5.1  1.2                                      UVT (PM)                                                                              19.7                                                                              2.1  13   13.8 13.8 15.8 6.1                                      a*      -7.2                                                                              +13.3                                                                              -1.2 +3.2 -11.6                                                                              -12.5                                                                              -10.3                                    b*      -2.5                                                                              +35.7                                                                              +15.9                                                                              +21.9                                                                              -8.2 -11.4                                                                              -5.4                                     dom λ nm                                                                       490.4                                                                             584.1                                                                              575  578.8                                                                              487.5                                                                              486.3                                                                              489                                      Colour Purity %                                                                       7.5 67.1 21.8 30.7 18.3 22.1 16.9                                     log4 visc.                 976 W                                              temp. °C.           970 Di                                             Liquidus                                                                      Temp. (T.sub.l)                                                               T.sub.w - T.sub.l          2.4961                                                                             2.4955                                                                             2.4967                                   Densities                                                                             2.4944                                                                            2.4940                                                                             2.4949                                                                             2.4944                                                  Refractive Index                                                              __________________________________________________________________________    Example      29     30     31      32                                         __________________________________________________________________________    SiO.sub.2    72.6   72.6   70.6    70.6                                       Al.sub.2 O.sub.3                                                                           0.1    0.1    0.91    0.91                                       Na.sub.2 O   13.9   13.9   13.7    13.7                                       K.sub.2 O    0.03   0.03   0.57    0.57                                       MgO          4.1    3.5    4.0     3.4                                        CaO          8.9    7.3    9.76    8.16                                       SO.sub.3     0.23   0.23   0.27    0.27                                       Fe.sub.2 O.sub.3                                                                           0.11   2.2    0.1     2.2                                        Ferrous state                                                                              20%    24%    20%     24%                                        Co.sub.3 O.sub.4    275            275                                        Se                  23             22                                         NiO                 250            250                                        III. A Trans        9.5            9.3                                        DSHT                6.8            6.5                                        UVT (ISO)           0.8            0.6                                        UVT (PM)            4.4            3.6                                        a*                  -6.9           -7.2                                       b*                  +2.1           +3.8                                       dom λ nm     507.7          530                                        Colour Purity %     4.2            5                                          log4 visc.   1014   1020   1000    1010                                       temp. °C.                                                              Liquidus     1008 W 996 Si 1028 W  990 W                                      Temp. (T.sub.l)     988 W                                                     T.sub.w - T.sub.l                                                                          +6            -28     +20                                        Densities    2.5017 2.5017 2.5176  2.5237                                     Refractive Index                                                                           1.5201        1.5249                                             __________________________________________________________________________

In Table 1, there are shown the compositions and spectral properties ofa variety of glasses in a 2 mm thickness. In Table 2, the spectralproperties of the glasses having the compositions shown in Table 1 aregiven, but at a thickness of 4 mm.

In Table 1, Example 1 is an example of a clear float glass containing anamount of iron oxide at an impurity level. Examples 2a and 2b areglasses containing 1.2 weight percent Fe₂ O₃ added to the basiccomposition of Example 1. In Example 2a, in accordance with one of theknown techniques described hereinbefore, the total weight percentage ofthe composition is now notionally 101.2 and this must obviously bere-calculated to bring the total back to 100%. Example 2b shows asimilar composition to Example 2a but, in this case, the extra 1.2weight percent Fe₂ O₃ has been added at the expense of SiO₂. All of theother Examples up to and including Example 28 with the exceptions ofExample 8 (similar to Example 2a), Example 15 (in which the iron oxidehas been added at the expense of MgO rather than CaO) and Example 16 (inwhich all of the MgO and some CaO has been removed and has been replacedby the iron oxide) are compositions for use in the present invention inwhich the densities of the iron oxide containing glass has been balancedwith the densities of the clear glass shown in Example 1. Examples 9 to11 and 13 to 17, in both Tables, are green grey glasses; Examples 18 to22 are grey glasses; Examples 23 to 25 are bronze glasses and Examples12 and 26 to 28 are blue glasses. As mentioned hereinbefore, there is no"standard" clear glass composition because, for example, the various rawmaterials used for making glass have varying compositions depending uponthe source from which they are obtained or the glass manufacturer has aparticular composition which he prefers to use. Accordingly, Examples 29and 31 show clear glasses having different compositions from that shownin Example 1 and Examples 30 and 32 show tinted glasses, which aregrey-green, having densities matched to those of these two clear glassesrespectively.

From the Tables, it will be clearly seen that the densities of thetinted glasses are closely matched with the corresponding clear glass.Moreover, the working range, that is to say, the difference the workingtemperature and the liquidus temperature has been increased. We havefound that, by carrying out the method of the present invention, thechangeover from making a clear glass to a tinted glass can be shortenedby hours and the changeover from tinted glass to clear glass can beshortened by days. Furthermore, from the spectral values given, it canclearly be seen that glasses having the requisite luminous, infra-redand ultraviolet transmissions can be obtained.

Various modifications may be made to the method of the present inventionwithout departing from the scope thereof Thus, for example, although theabove description is primarily concerned with effecting a changeoverfrom clear glass to tinted glass and vice versa, the method may equallyeasily be used for effecting a changeover between two different tintedglasses.

We claim:
 1. A method for making different glasses, which comprises:(1)supplying a glass making composition to a glass making furnace andforming a first glass; and (2) changing the glass making compositionthat is being supplied to the glass making furnace by interchanging ironoxide and alkaline earth metal oxide in the composition so that thedensity of the glass produced after said change in glass makingcomposition is within 0.01 g/cm³ of the density of the glass producedbefore said change in glass making composition to form a second glass.2. A method of making glass as recited in claim 1 wherein at least oneof said first and said second glasses is a clear glass.
 3. A method asrecited in claim 1 wherein at least one of said first and said secondglasses is a tinted glass.
 4. A method as recited in claim 1 whereinboth of said glass making compositions form tinted glass containing ironoxide.
 5. A method as recited in claim 4 wherein at least one of saidtinted glasses contains at least 0.7 weight% iron.
 6. A method asrecited in claim 4 wherein at least one of said tinted glasses containsat least 2 weight% iron.
 7. A method as recited in claim 1 wherein oneof said first and second glasses is a clear glass soda-lime-silicacomposition of SiO₂, Na₂ O, CaO, MgO, K₂ O and Al₂ O₃ and the other ofsaid first and second glasses is a tinted glass which includes at leastFe₂ O₃, CaO, SiO₂, Na₂ O, MgO, K₂ O and Al₂ O₃.
 8. A method as recitedin claim 7, wherein the concentration of CaO and MgO are each higher insaid clear glass than in said tinted glass whereby the densities of thetwo glasses are maintained substantially equal to one another.
 9. Amethod as recited in claim 4 wherein both said first and second glassescontain Fe₂ O₃, one of said first and said second glasses containingmore Fe₂ O₃ than the other of said first and second glasses, wherein thedensities of the two glasses are maintained substantially equal byreducing the amount of CaO in said glass containing the greater amountof Fe₂ O₃ relative to the amount of CaO present in said glass containingthe lesser amount of CaO.
 10. A method as recited in claim 1 wherein oneof said first and said second glasses contains:

    ______________________________________                                               A%          CaO                                                               B%          MgO                                                               C%          Na.sub.2 O + K.sub.2 O                                            D%          SiO.sub.2 + Al.sub.2 O.sub.3                               ______________________________________                                    

and the other of said first and said second glasses contains:

    ______________________________________                                        X%                     Total Iron                                                                    (expressed as Fe.sub.2 O.sub.3)                        (A - 0.5 X) % to  (A + 0.5 X) %! (A - 1.5 X) %                                                       CaO                                                    B% to (B - X) %        MgO                                                    C% to (C + 0.5 X) %    Na.sub.2 O + K.sub.2 O                                 D%                     SiO.sub.2 + Al.sub.2 O.sub.3                           ______________________________________                                    

while the proviso that the amount of MgO tends to (B-X)% when the amountof CaO tends to (A-0.5X)%! (A-1.5X)%.
 11. A method as recited in claim10 wherein A is from 5% to 9.5%, B is from 0% to 5%, C is from 10% to18%, D has a maximum value of 75% with the amount of SiO₂ lying withinthe range of 68% to 73% and the amount of Al₂ O₃ lying within the rangeof 0% to 5%, and X lies within the range of from 0.7% to 5%.
 12. Amethod as recited in claim 1 wherein the difference in density betweensaid first glass and said second glass is less than 0.005 g/cm³.
 13. Amethod as recited in claim 1 wherein each of said glass has a refractiveindex and said refractive indices differ by less than 0.004.
 14. Amethod as recited in claim 4 wherein the iron content, measured as Fe₂O₃, of said compositions producing said first and said second glassesdiffers by at least 1%.
 15. In a method for changing the glass producedfrom a continuous glass making process, which comprises continuouslysupplying a glass making composition to a glass making furnace andforming glass, the improvement which comprises:changing the glass makingcomposition, which contains iron oxide or calcium oxide or both, byinterchanging iron oxide and calcium oxide in the composition so thatthe density of the glass produced after said change in glass makingcomposition is within 0.01 g/cm³ of the density of the glass producedbefore said change in glass making composition.
 16. The method accordingto claim 15, wherein iron oxide is interchanged with calcium oxide andmagnesium oxide so that the density of the glass produced after saidchange in glass making composition is within 0.01 g/cm³ of the densityof the glass produced before said change in glass making composition.